Fabrication of heat exchange devices



Sept. 20, 1966 T. F. PAULS FABRICATION OF HEAT EXCHANGE DEVICES I 5 Sheets-Sheet 1 Filed June 12, 1965 INVENTOR THERON l-T PAULS mawi p MM FIG-1 ATTORNEY Sept. 20, 1966 T. F. PAULS 3,273,227

FABRICATION OF HEAT EXCHANGE DEVICES Filed June 12, 1963 5 Sheets-Sheet 5 INVENTOR. THERON F PAULS BY 717 0. Mm

A TTORNE V United States Patent Ofi ice 3,273,227 Patented Sept. 20, 1966 3,273,227 FABRICATION OF HEAT EXCHANGE DEVICES Theron F. Pauls, Madison, IlL, assignor to Olin Mathieson Chemical Corporation, a corporation of Virginia Filed June 12, 1963, Ser. No. 287,240 3 Claims. (Cl. 29-1573) This invention relates generally to the fabrication of heat exchange devices and more particularly to single piece sheet metal heat exchange structures having secondary heat dissipating iins integrated therewith.

A commonly used and efficient type of heat exchange unit for evaporators, air conditioning, condensers, internal combustion engine cooling radiators and the like is formed from a plurality of superimposed sheets of metal and having internally disposed between the sheets a number of conduits generally in a parallel spaced arrangement extending from a first or intake header to a second or outlet header. One or more of such units may be employed; and the conduits or tubes serve to carry a heat exchange medium such as water or other coolant in conductive relationship with another medium such as air or other gas passing between the tubes. This type of construction is typical of automobile radiators where, for example, the heated water issues from the cooling block of the engine with the aid of a pump, first enters one of the two headers, and then passes through a great number of thin-walled, relatively flat, closely spaced tubes between which cooling air is blown and which extend usually vertically from one to the other of the headers. Condensers are also frequently of this same type of construction.

According to one known method of manufacture as illustrated in US. Patent 2,690,002, this type of heat exchange unit may be readily manufactured to provide a great multiplicity of tubes in a sheet of metal. This method involves the application of a suitable predetermined pattern of weld inhibiting material between component sheets, pressure welding all adjoining areas except those separated by the weld inhibiting material, thereby forming a unified composite panel, and inflating along the un-welded areas to erect the tubes integral with the resultant tubed panel. Full advantage heretofore has not been taken of this method inasmuch as the tubes formed are of rather flat or oval shape with the major dimensions lying within or parallel to the panel in which the tubes are formed. In many applications it is desirable that the tubes extend not only longitudinally but also extend perpendicularly out of the panel to a considerable extent so as to place a greater number of the tubes in spaced parallel relationship rather than a lesser number in the same plane. This design adapts the units to fabrication as single pieces of large size, a lesser number of which may then be put together for installations where the external medium passes through perforations in the panel transversely to it rather than passing parallel to the panel along its surfaces.

In accordance with one aspect of this invention a sheet metal panel is formed according to the procedure of the above mentioned US. Patent No. 2,690,002, toform the desired tubular passageway system in its embryonic form. This panel is then slit along a plurality of parallel spaced apart lines in an area extending between two oppositely disposed headers to define the interconnecting tubes this area having been pressure rolled to effect a reduction in thickness between this area and the area of the panel coriesponding to the headers. The tube portions lying between adjacent slits are the bent or twisted out of the normal plane of the panel so as to dispose the tube portions in substantially perpendicular relationship to the plane of the panel.

In order to improve the heat transfer characteristics of the device, secondary heat dissipating fin material is inserted b6lItW6Il the parallel opposing surfaces or rolls of adjacent tubes and secured in place as by brazing or soldering. This construction, While extremely simple to fabricate and assemble, presents a practical and highly efiicient heat exchanger adapted to provide a maximum amount of external heat exchange medium flow between the tubes with a minimum amount of turbulence or impediment thereto.

Having thus generally described my invention it becomes a principal object thereof to provide a method of fabricating a compact and highly efficient heat exchange device adapted for transfer of heat between an internal and external heat exchange medium.

Another object of the present invention is to provide a method of fabricating a heat exchange device having a plurality of parallel heat transfer tubes interconnected between a pair of headers for maximum flow of an internal heat transfer medium.

Still another object of the present invention is to provide a method of fabricating a heat transfer device having a plurality of heat transfer tubes interconnected between a pair of headers, the tubes being elongate in cross section and of a thickness less than the thickness of said headers, with the cross sectional major dimension of the tube being disposed at substantially right angles to the normal plane of the panel from which the device is fabricated.

Still another object of the present invention is to provide a method of fabricating a heat'exchange device having a plurality of heat transfer tubes interconnected between a pair of oppositely disposed headers which are bent or twisted out of the normal plane of the panel from which the device is fabricated so as to provide slots or apertures through which an external heat transfer medium may flow unimpeded over the external surfaces of the heat transfer tubes.

It is still a further object of the present invention to provide a method of fabricating a heat exchange device having a plurality of heat transfer tubes interconnected between a pair of headers and having the major cross sectional dimension disposed at substantially right angles to the normal plane of the panel from which the device is fabricated to provide elongate apertures in which secondary heat dissipating fin stock material is inserted in 'heat transfer relationship with the outer .walls of the tubes to achieve maximum efficiency of heat exchange between the internal and external heat transfer mediums.

Yet another object of the present invention is to provide a method of fabricating a heat exchange device for use as automobile radiators, cooling system condensers and evaporators and the like, which is highly efficient, compact, and economical to manufacture.

Further objects and advantages of the present invention will become apparent from the following detailed decription when considered in conjunction with the accompanying drawings, in which:

FIGURE 1 is a plan view of one embodiment of the completed heat exchange device of this invention;

FIGURE 2 is a composite plan view illustrating a number of steps involved in the fabrication of the device of FIGURE 1;

FIGURE 3 is a sectional View taken on line 3-3 of FIGURE 2;

FIGURE 4 is a view similar to FIGURE 3 illustrating the device in an intermediate stage of fabrication;

FIGURE 5 is a fragmentary view similar to FIGURE 4, but on an enlarged scale, illustrating a further stage in the fabrication of the device;

FIGURE 6 is a fragmentary sectional view on an enlarged scale taken on line 66 of FIGURE 1;

FIGURE 7 is a fragmentary section view on an enlarged scale taken on line 7--7 of FIGURE 1;

FIGURE 8 is a fragmentary view illustrating an alternate embodiment of the invention; and

FIGURE 9 is a sectional view similar to FIGURE 4 taken on line 99 of FIGURE 2 and illustrating a modification of this invention.

Referring now to the drawings and particularly to FIG- URE 1, there is seen an illustrative embodiment of this invention which is a heat exchange device generally indicated by the reference numeral 10. The initial stage of fabrication of this device is substantially as set forth in great detail in the above mentioned U.S. Patent 2,690,- 002, and is generally illustrated, in conjunction with other steps in the formation of the heat exchange device 10, in FIGURES 2, 3 and 4.

Referring now to FIGURE 2 it will be seen that the heat exchange device 10 is initially formed from a plurality of superposed fiat metal sheets 12 and 14. Sheet 12 has applied thereto a pattern of weld preventing material 16 which is a foreshortened version of the desired pattern of tubular passageways in the finished article. This pattern consists of a pair of parallel bands 18 and 20 which are spaced apart adjacent a pair of opposite edges of the stack of sheets formed by the individual sheets 12 and 14. Interconnecting the two bands 18 and 20 are a plurality of bands 22 of weld preventing material which cover the extent of sheet 12 intermediate bands 18 and 20 except for elongated parallel islands 24 which are free of weld preventing material, and which also extend between the aforementioned bands 18 and 20. It will become apparent that the bands 18 and 20 of weld preventing material correspond to the headers in the finished article and that the bands 22 correspond to the plurality of interconnecting tubes. In order to provide ingress and egress apertures for heat transfer medium, the bands 18 and 20 are extended to an edge of sheet 12 as indicated at 26. It will also be seen that a marginal portion of sheet 12 along opposite sides transverse to the aforementioned opposite sides has been left free of weld preventing material 16 and is surrounded by a peripheral marginal area 28 with the exception of the two strips 26 extending to one of the transverse edges for the ultimate provision of openings adapted for connection to an external source of heat transfer medium.

The stack of component sheets 12 and 14 with weld preventing material 16 sandwiched therebetween is then temporarily secured together as by clamps, spot welding or the like to prevent relative movement between the sheets 12 and 14. The assembly thus formed is then heated to a required temperature and fed through a pair of pressure rolls which exert suflicient pressure on the stack to firmly weld the sheets 12 and 14 together into a single integrated sheet in the areas not coated with the weld preventing material 16. Simultaneously with the bonding operation the sheets 12 and 14 undergo a substantial reduction in thickness as well as an elongation in the direction of rolling whereby the foreshortened pattern of weld preventing material is stretched to a length corresponding to the desired pattern of tubular passageways in the finished article. FIGURE 4 illustrates in cross section the unified sheet 30 with the unwelded portions 32 at this stage of the fabrication.

Referring again to FIGURE 2 it will be seen that a plurality of slits 34 are formed in the islands 24 free of weld preventing material, the slits extending almost the full length of these islands. An additional slit 36 is provided in the transverse marginal area free of weld preventing material for a purpose hereinafter to become apparent. The slits 34 and 36 may be formed during any desirable stage of the fabrication process, either in the individual sheets 12 and 14, or after the sandwich of sheets 12 and 14 and weld preventing material 16 has been formed and temporarily secured together or still alternatively after the aforementioned hot rolling step.

Preferably the last mentioned alternative would be selected so as to eliminate both the problem of proper alignment of individual sheets 12 and 14 with slits already formed therein, and the problem of rewelding of adjacent slit edges during the hot rolling step if no weld preventing material is inserted between these edges. However, it is apparent that the article is readily adaptable to any of a number of arrangements of the aforementioned steps to achieve the heat exchange device in its embryonic form as illustrated in cross section in FIGURE 4.

A still further alternative to the formation of the slits is to form the panel in embryonic form as seen in FIG- URE 4 but without the slits, subject it to the foregoing hot rolling, and then as more fully explained hereinbelow, inflate the unjoined portions of the panel defined by the areas of weld preventing material. Subsequent to this, the inflated panel is subjected to a combination shearing and turning tool which simultaneously parts the metal to form the slits and rotates the inflated tubes.

Subsequent to the pressure welding stage in the fabrication process, the unwelded areas 26 which extend to the traverse edge of the unified sheet 30 are mechanically pried open and a suitable nozzle is inserted therein and connected to an external source of pressure fluid. The pressure fluid is pumped into the unwelded portions defined by the weld preventing material 16 to expand the sheets 12 and 14 over these portions and thereby create the desired system of internal fluid passageways. The expansion may be carried out either without external restraint thereby resulting in passageway walls having a rounded configuration, or preferably the expansion may be carried out with the composite sheet 30 inserted between suitable dies or plattens, either flat or shaped, so as to limit the extent to which the passageway walls can expand outwardly, thus creating a tubular passageway of generally rectangular configuration in cross section as seen in FIGURE 5. The heat exchange device now consists essentially of the flat unified sheet 30 with an expanded pattern of tubular passageways corresponding to the original pattern of weld preventing material 16, With the bands 18 and 20 forming the headers 40 and 42.

Referring again to FIGURE 1, the heat exchange device 10 comprises the integrated sheet 30 of generally rectangular configuration, having the parallel spaced apart internally disposed headers 40 and 42 and the elongate tubular passageways 3-8. The headers 40 and 42 terminate adjacent an edge of sheet 30 in inlet and outlet openings 44 and 46 respectively to which conduits 48 and 50 respectively are connected for communication of the heat exchange device with a source of internal heat transfer medium.

It will be seen from FIGURE 1 and in greater detail in FIGURE 6 that the tubular passageways 3-8 have been twisted out of the normal plane of sheet 30 so as to dispose the major cross sectional dimension of tubes 38 at approximately right angles to the normal plane of sheet 30. This disposition of tubes 38 is accomplished by any desirable means, and has an extent covering substantially all of the length of the slits 34 previously formed in the unified sheet 30. As indicated above, the slitting may occur simultaneously with the tube rotation by the use of a combination tool. Thus in the finished product the individual tubes 38 are disposed with opposed parallel outer wall surfaces 52 in spaced apart relationship, providing relatively wide and elongate slots or apertures 54 through which an external heat transfer medium can flow substantially unimpeded. Communication between the headers 40 and 42 and interconnecting tubes 38 is maintained through the interior of the tubes 38 which traverses the transition portions 56 of tubes 38 which lie between the point of connection between the headers '40 and 42 and the remainder of the tubes 38 which are disposed in the aforementioned perpendicular relationship. Of course, it is desirable to maintain these transition portions 56 as short as possible within the limits of the bending characteristic of the metal in order to provide the maximum length of apertures 54 for unimpeded flow of air and, as will be seen in more detail hereinbelow, to provide the maximum amount of space for secondary heat dissipating fins.

During rotation of the tubes, these transition portions 56 may be allowed to collapse and be reinflated in the weakened state during a subsequent brazing operation, more fully explained below. In the alternative, it may in some instances be desirable to insert in the headers 40 and 42 a tool similar to a comb and insert the pins or teeth thereof into the tubes 38 and rotate them around these pins or teeth, thereby preventing collapse.

It should be noted that, as with the aforementioned slitting step to provide the slits 34 and 36, it is alternative whether the twisting step to dispose the tubes 38 out of the normal plane of the unified panel 30 is carried out before or subsequent to the inflation step, or simultaneously therewith. One advantage to performing the twisting step after inflating the embryonic panel 30 lies in obviating the inherent difficulties involved in placing the panel 30 between the aforementioned dies or plattens with the tubes 38 already bent at right angles to the plane of the remainder of panel 30. Also slitting and rotating subsequent to inflating eliminates the problem of identifying the location on the panel of the islands 24 in which the slits are to be made.

Whatever the sequence of the aforementioned events, when the tubular passageways have been completed and the interconnecting tubes 38 bent to the desired relation ship secondary heat dissipating fins 58 formed of closely corrugated or pleated fin stock may be inserted and positioned within the elongate spaces or apertures between the opposing faces 52 and tubes 38, and suitably secured therein by conventional means, such as solder, brazing and the like interposed between the panel and the fin stock. It is also possible to insert the fins before the tubes are fully inflated so that they will easily drop into place, after which additional pressure is applied during the brazing operation to insure intimate contact of the fins with the tubes. In one form this fin stock may be formed by bending strips of highly heat conductive metal into a corrugated or serpentine form. It will be noted from FIGURE 6 that when the tubes 38 are bent, the welded portions of sheets 12 and 14 formerly defined by the islands 24 free of weld preventing material now constitute oppositely directed flanges 60, thereby leaving the opposing faces 52 of tubes 38 smooth and free of any projecting obstructions. Consequently, it is a relatively simple matter to slip the strips of fin stock 58 into place between adjacent tubes for subsequent unification therewith as explained above. In addition the flanges 60 add structural rigidity to the device and also reinforce the leading edge of the tubes for minimizing damage thereto due to stones or other foreign objects which can strike the frontal area of an automotive radiator. Also the flanges constitute additional heat transfer stock.

In order to secure the outermost rows of fin stock 62 and 64 in place, outer retaining strips 66 and 68 are formed by providing the two outermost slits 36 (FIGURE 1) beyond the pattern of weld preventing material 16. Thus, during the hot rolling process a strip of sheets 12 and 14 is bonded together between the outermost slits 36 and adjacent slits 34 which, when twisted in the same manner as tubes 38, form the outer retaining strips 66 and 68 respectively.

In the manufacture of automobile radiators and the like, a. plurality of the resultant blanks 30 may be assembled in face to face relationship with the tubes and the elongate spaces or apertures in alignment with the corresponding tubes and apertures in adjacent panels, or the tubes of one panel may be in overlapping relationship with tubes of adjacent panels, wherein secondary fin stock is individually inserted in each panel prior to assembly. In a still alternate procedure, the plurality of panels may be assembled in face to face relationship prior to secondary fin stock insertion wherein the tubes and the apertures of one panel are in alignment with the corresponding tubes of the adjacent panels. Thereafter, app1 opriate secondary fin stock may be inserted in the apertures between tubes, so that the fin stock will be coextensive with the assembled plurality of panels, that is the fin stock extends through all of the panels.

Several embodiments of the foregoing structure and alternate methods of fabrication are apparent. One such modification is illustrated in FIGURE 8 and has for its purpose a closer spacing between the tubes interconnecting the spaced apart headers, thereby minimizing the amount of space required for a given unit with a predetermined amount of heat exchange ability, either with or without secondary fin stock material. To achieve this configuration the device is initially fabricated with the tubes spaced, for example, on approximately 1 /2" centers; the tubes are then rotated as in the foregoing embodiment, after which the headers are collapsed between the tubes such as is done in bellows-type flexible metal tubing, etc. This produces corrgulations or undulations in the headers having the ridges and valleys 70 and 72 respectively such as in FIGURE 8. The tubes, which originally were on the 1 /2," centers, would be brought down to approxi mately to /2 centers, or other predetermined spacing, after which the secondary fin stock could be inserted between the tubes, if desired, as described above. This also would tend to strengthen the headers which would be of the same wall thickness as the tubes.

A second modification in the method here disclosed provides for a diiferential in wall thickness between the headers and the tubes. This may be achieved, for example, by the use, during the initial rolling process to form the unexpanded panel, of a stepped roll which would roll the tube thickness from say .030" down to .010 prior to rotating the tubes. This results in a reduced gauge tube while keeping a heavy Wall in the headers.

This relationship is illustrated in FIGURE 9, which depicts the device in an intermediate stage of production. Following the procedure described above, the assembly depicted in FIGURE 2. is temporarily secured together and heated in the manner indicated hereinbefore, and then fed through a pair of stepped pressure rolls to (1) exert suflicient pressure to weld the sheets 12 and 14 together in the areas not coated with the weld-preventing material 16, and (2) to effect the desired decrease in thickness in the area of the tubes, relative to the header sections.

Thus, considering a section of such a unified sheet 30 taken on the line 99 of FIGURE 2, it will be seen that the thickness of the panel adjacent the bands 22 has been substantially decreased with respect to the thickness of the panel adjacent the bands 18 and 20. Following further processing identical to that indicated for sheet 30 of FIGURE 4, the resulting modified panel is substantially identical to that depicted in FIGURE 6, except that the thickness of the tubes would be reduced to, for example, one-third that illustrated in FIGURE 6.

'It is apparent that this procedure yields a heat exchange device substantially of the type illustrated in FIGURE 1 having the desired differential in thickness between the headers and tubes. The panel, the area of the tubes being reduced to a thickness approximately onethird the thickness of the headers, would be of a length between the headers approximately three times that of the panel illustrated in FIGURE 1. Accordingly, the finished article employing this procedure would be of a length approximately three times that which otherwise obtained. Accordingly, greater heat transfer contact area between the tubes and the external heat exchange medium is achieved, while at the same time maintaining a higher degree of structural rigidity in the header portions of the device and greater support for the tubes therebetween.

It should be noted that with any of the above embodiments and alternate methods of fabrication, as much cooling capacity as required may be achieved by arranging the heat exchange units in multiples with the headers of the several units being connected in parallel.

It will be apparent from the foregoing description and accompanying drawings that there has been provided a heat exchange device and method for making the same which is believed to provide a solution to the foregoing problems and achieve the aforementioned objects. It is to be understood that the invention is not limited to the illustrations described and shown herein which are deemed to be merely illustrative of the best modes of carrying out the invention, and which are susceptible of modification of form, size, arrangement of parts, and detail of operation, but rather is intended to encompass all such modifications as are within the spirit and scope of the invention as set forth in the appended claims.

What I claim and desired to secure by Letters Patent is:

1. A method of fabricating a heat exchange device comprising the steps of (A) providing a composite sheet formed of two superposed planar sheets having a pattern of Weld preventing material interposed between said sheets, said composite sheet having opposed first edges and opposed second edges, said pattern including a pair of parallel spaced apart bands located adjacent said first edges and extending to one of said second edges and a plurality of closely spaced apart bands lying parallel to said second edge and joined at opposite ends with said pair of bands, said pair of bands and plurality of bands thereby defining islands free of said weld preventing material,

(B) pressure rolling said composite sheet in a direction parallel to said closely spaced apart bands to unify said composite sheet except in the areas of said pattern of weld-preventing material and to effect a differential in reduction of thickness of said composite sheet between the intermediate portion thereof defined by said plurality of bands of weld preventing material and material free islands, and the remaining outer portions of said composite sheet, with said intermediate portion being subjected to a greater reduction than said outer portions,

(C) slitting said composite sheet in said weld preventing material free islands over a major portion of the length thereof to provide a plurality of individual strips of said composite sheet,

(D) bending said strips adjacent the opposite end portions thereof in a rotary direction relative to the longitudinal axis of said strips by approximately to displace the elongate portion of said strips between said end portions out of the normal plane of said composite sheet, thereby forming elongate apertures between adjacent confronting surfaces of said strips, and

(E) forming a system of internal passageways within said composite sheet corresponding to said pattern of Weld preventing material by applying thereto a fluid under pressure to expand the areas of said composite sheet covered with said weld preventing mate rial.

2. A method as set forth in claim 1 further including the steps of inserting a corrugated strip of secondary heat exchange fin stock into said apertures between said confronting surfaces and securing said fin stock to said surfaces.

3. A method as set forth in claim 1 further including the step of collapsing in corrugated tube fashion the portions of said composite sheet adjacent to and surrounding said pair of spaced apart bands of weld preventing material, thereby reducing the center-to-center spacing of said strips.

References Cited by the Examiner UNITED sTATEs PATENTS 2,190,490 2/1940 Sendzimer 29-1573 2,759,247 8/1956 01611611 et al. 29-1573 2,779,086 1/1957 Rieppel et a1. 29 157.3 2,856,164 10/1958 Adams 29-45713 X 2,894,731 7/1959 Wurtz 29 157.3 X 2,926,003 2/1960 P6151161 29-157-3 X 2,932,491 4/1960 Miller --148 2,957,679 10/1960 Campbell 29-1573 X 2,982,013 4/1961 Adams 29-1 573 2,998,639 9/1961 Forst et al. 29-1573 2,999,308 9/1961 Pauls 29-1573 FOREIGN PATENTS 214,980 5/1958 Australia. 1,238,450 7/1960 France.

813,589 5/1959 Great Britain.

JOHN F. CAMPBELL, Primary Examiner.

CHARLES SUKALO, WHITMORE A. WILTZ, T. W.

STREULE, J. D. HOBART, Assistant Examiners. 

1. A METHOD OF FABRICATING A HEAT EXCHANGE DEVICE COMPRISING THE STEPS OF (A) PROVIDING A COMPOSITE SHEET FORMED TO TWO SUPERPOSED PLANAR SHEETS HAVING A PATTERN OF WELD PREVENTING MATERIAL INTERPOSED BETWEEN SAID SHEETS, SAID COMPOSITE SHEET HAVING OPPOSED FIRST EDGES AND OPPOSED SECOND EDGES, SAID PATTERN INCLUDING A PAIR OF PARALLEL SPACED APART BANDS LOCATED ADJACENT SAID FIRST EDGES AND EXTENDING TO ONE OF SAID SECOND EDGES AND A PLURALITY OF CLOSELY SPACED APART BANDS LYING PARALLEL TO SAID SECOND EDGE AND JOINED TO OPPOSITE ENDS WITH SAID PAIR OF BANDS, SAID PAIR OF BANDS AND PLURALITY OF BANDS THEREBY DEFINING ISLANDS FREE OF SAID WELD PREVENTING MATERIAL, (B) PRESSURE ROLLING SAID COMPOSITE SHEET IN A DIRECTION PARALLEL TO SAID CLOSELY SPACED APART BANDS TO UNIFY SAID COMPOSITE SHEET EXCEPT IN THE AREAS OF SAID PATTERN OF WELD-PREVENTING MATERIAL AND TO EFFECT A DIFFERENTIAL IN REDUCTION OF THICKNESS OF SAID COMPOSITE SHEET BETWEEN THE INTERMEDIATE PORTION THEREOF DEFINED BY SAID PLURALITY OF BANDS OF WELD PREVENTING MATERIAL AND MATERIAL FREE ISLANDS, AND THE REMAINING OUTER PORTIONS OF SAID COMPOSITE SHEET, WITH SAID INTERMEDIATE PORTION BEING SUBJECTED TO A GREATER REDUCTION THAN SAID OUTER PORTIONS, FEIG-01 (C) SLITTING SAID COMPOSITE SHEET IN SAID WELD PREVENTING MATERIAL FREE ISLANDS OVER A MAJOR PORTION OF THE LENGTH THEREOF TO PROVIDE A PLURALITY OF INDIVIDUAL STRIPS OF SAID COMPOSITE SHEET, (D) BENDING SAID STRIPS ADJACENT THE OPPOSITE END PORTIONS THEREOF IN A ROTARY DIRECTION RELATIVE TO THE LONGITUDINAL AXIS OF SAID STRIPS BY APPROXIMATELY 90* TO DISPLACE THE ELONGATE PORTION OF SAID STRIPS BETWEEN SAID END PORTIONS OUT OF THE NORMAL PLANE OF SAID COMPOSITE SHEET, THEREBY FORMING ELONGATE APERTURES BETWEEN ADJACENT CONFRONTING SURFACES OF SAID STRIPS, AND (E) FORMING A SYSTEM OF INTERNAL PASSAGEWAYS WITHIN SAID COMPOSITE SHEET CORRESPONDING TO SAID PATTER OF WELD PREVENTING MATERIAL BY APPLYING THERETO A FLUID UNDER PRESSURE TO EXPAND THE AREAS OF SAID COMPOSITE SHEET COVERED WITH SAID WELD PREVENTING MATERIAL. 